An overvoltage protection component is a component that turns on when the voltage across it exceeds a given threshold, i.e. a breakdown voltage and generally designated as VBR. A first type of protection component is an avalanche diode type, with a current-voltage characteristic illustrated in FIG. 1. When the voltage across this component exceeds breakdown voltage VBR, the component becomes conductive. Ideally, the voltage across the component remains equal to VBR while the current increases. Indeed, as shown in FIG. 1, the characteristic is not vertical and the voltage across the component exceeds value VBR while the overvoltage is absorbed, i.e. a current I of strong value crosses the component.
A disadvantage of this type of component is that during the overvoltage absorption phase, the voltage across the component remains greater than or equal to breakdown voltage VBR, i.e. during this phase, the component has to absorb a power greater than VBR×I. This results in having to form a component of sufficiently large size, on the one hand, to minimize its internal resistance and thus the voltage there across during the overvoltage removal phase and, on the other hand, so that it can absorb the power associated with the overvoltage without being destroyed. Currently, for voltages VBR greater than 100 volts, for example, on the order of 300 volts, this results in component sizes greater than several cm2, for example, on the order of 10 cm2. Such components are however made in the form of a stack of diode chips, for example, a stack of fourteen elementary components, each having a surface area of 8.6×8.6 mm2 to reach a 430-V breakdown voltage. Such components may be expensive and bulky.
A second type of protection component is of breakover type, i.e. a Shockley diode type, or a gateless thyristor type. The current-voltage characteristic of a breakover component is illustrated in FIG. 2. When the voltage across the component exceeds breakdown voltage VBR, this voltage rapidly drops and then follows a substantially vertical characteristic 1.
An advantage of this second type of component may be that the power dissipated by the overvoltage in the component is low as compared with the power dissipated in a device of avalanche diode type, given that the voltage across the component is very low during the overcurrent flow. A disadvantage of this second type of component may be that, as long as there is a significant voltage across the component, the component remains on, the protection component only turning back off if the voltage across is such that the current in this component becomes smaller than a hold current Ih. For a protection component having its breakdown voltage VBR ranging from 50 to 1,000 volts, this hold current currently has a value in the range from 100 mA to 1 A according to the breakdown voltage of the component.
Accordingly, breakover type protection components may be reserved for circuits where these components are intended to protect a line having an operating voltage crossing zero values—this being true, in particular, for a data transmission line. As illustrated in FIG. 3, if a line L1 forming a power supply line connected to the output of a power supply device, such as a solar plant 10, for example, connected to an inverter 12, is desired to be protected, a breakover protection component can normally not be used since, after the occurrence of an overvoltage, for example, corresponding to a strike of lightning on line L1, the potential on line L1 remains positive and the protection component remains conductive.
As illustrated in FIG. 4A, after application of the overvoltage, voltage VDC at the output of power supply source 10 is short-circuited and a short-circuit current Isc flows therethrough. The source sees, between its terminals, internal resistance Ri and on-state resistance RD of the protection diode. A voltage VD=VDC(RD/(Ri+RD)) then exists across the protection diode.
FIG. 4B shows a portion of the characteristic curve of the diode corresponding to this specific case. In most practical configurations, potential VD corresponding to short-circuit current Isc is much greater than potential Vh corresponding to hold current Ih of the breakover component. As an example, for a 150-mA hold current Ih, voltage Vh may be in the order of 2 V. It is thus not possible, in principle, to use a breakover component to protect a direct current (DC) power supply line. Protection devices of avalanche diode type, which should have significant surface areas and thus a high cost, thus have to be formed.
FIG. 5 illustrates an example of a protection device. This device is described in French patent application No. 1352864 (and in the corresponding United States Patent Application Publication No. 2014/0293493), which is incorporated herein by reference in its entirety. The device comprises, between two terminals A and B, the parallel assembly of a breakover type protection diode D, a switch SW, and a circuit CONTROL for controlling switch SW.
Protection device of FIG. 5 operates as follows. In the idle state, switch SW is off. Terminals A and B are connected across a DC power supply line so that the protection is for example connected like diode D of FIG. 3. As long as the voltage between terminals AB remains lower than the breakdown voltage of breakover diode D, the protection device is non-conductive. When an overvoltage appears, the protection diode becomes conductive, which results in the configuration of FIG. 4A, that is, the power supply connected between terminals AB is shorted. Once the overvoltage has passed, diode D conducts a short-circuit current Isc such as defined in relation with FIG. 4A. At this time, the switch SW is turned on so that the current between terminals A and B is branched by switch SW. If on-state resistance Ron of switch SW is sufficiently low, and in particular if condition Ron×Isc<Vh is respected, the voltage between terminals AB becomes lower than voltage Vh, and breakover diode D blocks. Switch SW can then be turned back off.
According to a first approach, the control circuit comprises an overvoltage detector and automatically turns on switch SW for a determined time period, sometime after the overvoltage will have been detected, and then turns off switch SW after a determined time. According to another approach, the control circuit comprises circuitry for detecting the voltage across diode D. As long as this voltage is lower than VBR and higher than VD, the control circuit will remain inactive. Then, after a first voltage drop, the control circuit will determine whether the voltage across diode D is within a given range, corresponding to value VDC(RD/(Ri+RD)). The control circuit then determines the turning on and the turning off of switch SW.
The operation of the circuit of FIG. 5 is based on the fact that, when switch SW is in the conductive state, the voltage there across drops sufficiently to become lower than previously-defined value Vh. This implies that on-state resistance Ron of switch SW should be much lower than apparent resistance RD of diode D when the device is shorted. It should be understood that this makes it necessary to use a switch with a very low Ron, which is not always compatible with the desire to use low-cost switches, for example, small MOS transistors.
FIG. 6 illustrates an alternative approach of the device of FIG. 5, which may overcome this disadvantage. In FIG. 6, the protection further comprises, in series with breakover diode D between terminals A and B, an avalanche diode d having a breakdown voltage Vbr much smaller than breakdown voltage VBR of breakover diode D. The operation of the series assembly of breakover diode D and of avalanche diode d differs little, in terms of overvoltage absorption, from the operation of diode D alone. This time, when the overvoltage has passed and the line is shorted, condition Ron×Isc<Vh+Vbr just has to be satisfied, which enables to use a switch having a higher Ron than in the case of the assembly of FIG. 5.